The Duke has the same stator as all other 690s - Enduro (R), SMC, Supermoto, Rally Replica. Unfortunately there is no higher power version available that will fit, neither OEM nor aftermarket.

Work on reducing inefficiencies, like going to better all LED and/or HID lighting (obviously blinkers with a proper relay), replacing the stock regulator with a MOSFET one (Uller and RoninMoto have done a write up), using dual element heated grips instead of resistors, etc.